xen/x86: obtain upper 32 bits of video frame buffer address for Dom0

The hypervisor has been supplying this information for a couple of major
releases. Make use of it. The need to set a flag in the capabilities
field also points out that the prior setting of that field from the
hypervisor interface's gbl_caps one was wrong, so that code gets deleted
(there's also no equivalent of this in native boot code).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>

Link: https://lore.kernel.org/r/a3df8bf3-d044-b7bb-3383-cd5239d6d4af@suse.com


Signed-off-by: Juergen Gross <jgross@suse.com>